This invention relates to a method for manufacturing an electrically insulating formed channel member which is applied to the inner corners of a coil of an electrical apparatus such as a rectangular pancake coil primarily used in a shell type transformer reactor.
In FIGS. 1 and 2, the manner in which a substantially rectangular pancake coil 1 is electrically insulated is illustrated. In an electrical apparatus, in order to mechanically support the rectangular pancake coil 1 against the electromagnetic force of the electrical apparatus, the rectangular pancake coil 1 is supported and insulated by a plurality of electrically insulating spacers 3 disposed on both of the major surfaces of the coil and by and an electrically insulating formed channel member 2 disposed at an inner corner of the rectangular pancake coil 1. The channel member 2 comprises a channel portion 4b including a web section 2a and flange sections 2b, and a plurality of projection portions 4a extending radially outwardly from the flange sections 2b. The channel portion 4b is bent along the inner corner of the rectangular pancake coil 1 with the web section 2a located at the inner curvature of the corner. The channel member 2 is mounted at the corner of the rectangular pancake coil so that the flange sections 2band the projection portions 4a are in contact with both major surfaces of the pancake coil 1. The electrically insulating formed channel member 2 is made of a sheet of an electrically insulating material, generally press board. In the embodiment illustrated in FIG. 2, the flange sections 2b are in contact with the upper and lower major surfaces of the rectangular pancake coil 1.
The conventional method for manufacturing the electrically insulating formed channel member 2 is illustrated in FIGS. 3 to 6. In FIG. 3 a sheet of an electrically insulating material capable of being press formed, such as dry paper press board with a 6% to 8% moisture content, is cut into a rectangular sheet of predetermined dimensions to obtain a press board 5. Then, the press board 5 is provided with ripples 6 to form a rippled press board 16 as shown in FIG. 4. The rippled press board 16 thus formed is immersed in pure water contained in a vessel (not shown) until the board 16 contains the proper amount of moisture. The rippled press board 16 is then press-formed at an elevated temperature to form a channel member bent along the inner corner of the rectangular pancake coil 1 as shown in FIG. 5. The channel member thus formed has a substantially U-shaped cross section and has a web portion 2a and flanges 4. Finally, the formed channel member shown in FIG. 5 is cut to remove a portion of the material of the flanges 4 to provide a plurality of projection portions 4a extending from the radially outer edge of flange portions 4b as shown in FIG. 6. In the illustrated example, the projection portions 4a have a radial dimension of a, the flange portions 4bhave a width dimension of b, and the flange 4 of the channel member 16 before cutting, i.e., as shown in FIG. 5, has a width of a+b.
In the above described conventional manufacturing method, however, a problem arises that cracks 9 as shown in FIG. 7 form in the radially outer edges of the flanges 4 during the press-forming of the rippled press board 16 of FIG. 4 into the formed channel member 16 of FIG. 5. During the press-forming of the channel member 16, compressive forces, as shown by arrows 7, are applied to the inner edge of the flange 4 of the formed channel member 16. Tensile forces, as shown by an arrow 8, are applied in the outer edge of the flange 4. This tensile force shown by the arrow 8 may exceed the tensile strength of the press board, resulting in the cracks 9 at the radially outer edge of the flanges 4. These cracks 9 easily appear when the radius of curvature of the inner corner of the rectangular pancake coil 1 is relatively small or when the width dimension a+b of the flange 4 is relatively large.